The present invention relates to photographic printing systems. In particular, the present invention is an improved color balancing technique for use in a photographic printer.
Photographic printers produce color or black and white prints or transparencies from photographic film originals (generally negatives). High intensity light is passed through the film and imaged on the photosensitive print medium (film or paper). The photographic emulsion layers on the print paper or film are exposed and subsequently processed to produce a print or transparency of the scene contained in the film original.
A critical portion of a photographic printer is the exposure control, which controls the exposure of the photosensitive medium to ensure that the image on the photosensitive medium is properly exposed. The exposure control may utilize inputs from several different sources in order to determine the proper exposure. For example, most automatic printers use large area transmission density (LATD) sensors to sample light transmitted by the negative either prior to or during an exposure. Control of the exposure is determined using a method known as "integration to grey". In addition, many automatic printers include an automatic density correction (ADC) or color/density scanning station which scans the negative prior to printing and corrects the exposure in the event of a condition known as "subject failure". Finally, the operator may enter density correction signals from the operator control panel. Based upon some or all of these input signals, the exposure control determines the proper exposure for each of the color channels or for one black and white channel.
In most photographic printers, the color content of the light passed through the negative is adjusted by means of subtractive filters. Complicated, time-consuming, "color balancing" procedures have been used to properly position or select the filters so that the printing light has the proper spectral content. While the prior art color balancing techniques have been generally effective, improved techniques are desirable. In particular, new techniques are needed which use less print paper in color balancing tests, which are less subjective, are less complicated, and require less time.